Last week Dropbox announced plans to go public with the goal of raising $500 million at an estimated $10 billion market valuation.  Dropbox launched 11 years ago and has since grown to more than $1.1 billion in annual revenues. Revenues have grown 39% and 31% over the last two years. While growth remains strong, the slowing pace of growth is concerning to some investors.

One revealing detail from the Dropbox filing is that the company is generating its revenue from just 2.2% of its customers, meaning just 11 million of its 500 million registered users pay for the service.Simple math suggests that ARPU is about $100 per year or about $8 per month per user. As with any cloud SaaS provider, customer engagement is a critically important metric.

Data from the most recent Tech Adoption Index survey of 1,000 SMBs shows that Dropbox has the highest indicated use of cloud-tools at 31%. No surprise here since I think most of us have, or have had, a Dropbox account.

And while there is no shortage of cloud-storage options, with the likes of Google, Apple and Microsoft all offering similar solutions, Dropbox has arguably been the most adept at viral marketing. Dropbox’s greatest opportunity is not adding to its base of 500 million users but driving the paid user base upwards from its current scant 2.2% level.